Senktide

Heat stress disrupts reproductive function in cattle. In summer, high ambient temperature and humidity elevate core body temperature, which is considered to be detrimental to reproductive abilities in cattle. Neurokinin B (NKB) is a factor that generates pulsatile GnRH and subsequent LH secretion in mammals. Recent studies have reported that NKB-neurokinin 3 receptor (NK3R) signaling is associated with heat-defense responses in rodents. The present study aimed to clarify the role of NKB-NK3R signaling in thermoregulation in cattle. We examined the effects of an NK3R-selective agonist, senktide, on vaginal temperature as an indicator of core body temperature in winter and summer. In both seasons, continuous infusion of senktide for 4 h immediately decreased vaginal temperature, and the mean temperature change in the senktide-treated group was significantly lower than that of both vehicle- and GnRH-treated groups. Administration of GnRH induced LH elevation, but there was no significant difference in vaginal temperature change between GnRH- and vehicle-treated groups. Moreover, we investigated the effects of senktide on ovarian temperature. Senktide treatment seemed to suppress the increase in ovarian temperature from 2 h after the beginning of administration, although the difference between groups was not statistically significant. Taken together, these results suggest that senktide infusion caused a decline in the vaginal temperature of cattle, in both winter and summer seasons, and this effect was not due to the gonadotropin-releasing action of senktide. These findings provide new therapeutic options for senktide to support both heat-defense responses and GnRH/LH pulse generation.

The effects of various manipulations of brain 5-HT mechanisms on the behavioural responses induced by the selective NK-3 tachykinin agonist senktide in rodents were assessed. Senktide elicited wet dog shakes in the rat which were attenuated by the 5-HT1C/2 antagonist mianserin and the selective 5-HT2 antagonist altanserin. Senktide-induced forepaw treading was stereospecifically attenuated by the 5-HT1A + B antagonist (-)-alprenolol. Senktide also elicited chewing mouth movements and yawning, which were unaffected by mianserin, altanserin, (+)- or (-)-alprenolol, or the selective 5-HT3 antagonist ICS 205-930, but attenuated by the muscarinic antagonist scopolamine. Penile grooming elicited by senktide was attenuated by mianserin, but was unaffected by the other antagonists. Senktide-induced wet dog shakes were enhanced by the 5-HT reuptake inhibitors citalopram and fluoxetine, suppressed by the monoamine oxidase (MAO)-B inhibitor pargyline, but unaffected by the MAO-A inhibitor clorgyline. Forepaw treading was potentiated by citalopram and clorgyline, but not significantly altered by fluoxetine or pargyline. Depletion of 5-HT by p-chlorophenylalanine (PCPA) in the rat attenuated senktide-induced wet dog shakes and forepaw treading. Neither PCPA nor 5,7-dihydroxytryptamine affected senktide-induced behaviours in the mouse, but the degree of brain 5-HT depletion caused by these treatments in mice was relatively small. These findings indicate that stimulation of NK-3 tachykinin receptors by senktide results in a complex behavioural syndrome which is mediated by multiple 5-HT receptors, and dependent upon intact stores of endogenous 5-HT. Independent stimulation of brain cholinergic mechanisms by senktide is also confirmed.

The aim of this work was to evaluate the capability of lactoferrin- and antitransferrin-modified long circulating liposomes to deliver the hydrophilic peptide senktide, a selective NK3 receptor agonist unable to cross the blood brain barrier, to central nervous system by using an indirect method based on in vivo microdialysis studies to estimate the responsiveness of nucleus accumbens shell dopamine to senktide. To this purpose, senktide was encapsulated in different targeted and not-targeted stealth liposomes prepared using film hydration method. Formulations were characterized in terms of morphology, size distribution, zeta potential, encapsulation efficiency, and antibody presence on the liposome surface. In vivo microdialysis studies were performed injecting intravenously the senktide-loaded liposomes and comparing obtained dopamine levels with those found with the free senktide given intracerebroventricularly. Results showed that all vesicles were spherical, small in size (around 120 nm), homogeneously dispersed, and slightly negatively charged. TEM analysis, using an anti IgG secondary antibody with 10nm gold nanoparticles at its distal end, demonstrated the successful linkage of the antibody on the liposomal surface. Intravenously administered in rats, senktide-loaded targeted stealth liposomes elicited a significant increase of dialysate dopamine in the nucleus accumbens shell, which was comparable to that of the free senktide given intracerebroventricularly when antitransferrin-targeted liposomes were tested. On the contrary, control stealth liposomes did not affect dopamine levels. Senktide brain levels were higher using the antitransferrin-targeted liposomes in comparison with the lactoferrin ones, while the opposite was obtained in the liver tissue where the highest senktide accumulation was always found.